The present invention relates to a receiver for receiving encoded digital signals, said receiver comprises a front end for deriving words of digital symbols from an input signal, a de-interleaver for deriving de-interleaved words of digital symbols from said words of digital symbols, a decoder for deriving decoded words of digital symbols from said de-interleaved words of digital symbols and a burst error predictor for predicting possible symbol error positions in a de-interleaved word to be decoded from symbol error positions in at least one previously decoded de-interleaved word of digital symbols.
The present invention also relates to a decoder, a decoding method and a storage medium carrying a computer program for carrying out a decoding method.
A decoder according to the preamble is known from U.S. Pat. No. 5,299,208. Such receivers are used for receiving digital video signals or data signals from a transmission medium such as a satellite channel, a terrestrial channel or a CATV channel. In order to ensure a virtual error free transmission of the source symbols representing the video or data signals, channel coding is used. This channel coding often includes the use of a concatenated coding scheme. This concatenated coding scheme involves the encoding of the source symbols using an outer channel code, interleaving the encoded source symbols followed by encoding the interleaved symbols by an inner code. The outer code can be a block code such as a Reed-Solomon code, and the inner code is often a convolutional code.
The interleaving is used to decrease the vulnerability for burst errors, interleaving is used between outer coding and inner coding. The effect of interleaving is that burst errors are transformed into single error in multiple codewords. Because single errors are more easy to correct than burst errors, the use of interleaving will result in an increased performance on channels having bursty error characteristics.
The receiver for such a signal comprises a cascade connection of an inner decoder, a deinterleaver and an outer decoder.
In the above-mentioned U.S. patent the knowledge about the position of the single errors due to spread of burst errors is used to improve the quality of the decoder. Therefor this decoder comprises a burst error predictor that predicts the positions of possible errors from already occurred errors. This is possible by assuming that a burst error is present and using the known properties of the used interleaving scheme. The positions of all possible errors are passed to the outer decoder.
This outer decoder will regard the symbols at the indicated position as unreliable. E.g. in a Reed-Solomon decoder the indicated positions are treated as so-called erasures, errors of which the position is known. A Reed-Solomon decoder is able to correct n-1 random errors of which the position is not known, and 2n-1 errors (eraures) of which the position is known. This increase of error correcting capability results in an improved performance of the decoder.
A problem of the receiver according to the above-mentioned U.S. patent is that the numbers of unreliable symbols passed to the decoder can exceed its error correcting capabilities. In such a case, no decoding is possible at all. In the above U.S. patent, the structure of the interleaver is chosen such that the number of unreliable symbols is always limited to the error correcting capabilities of the decoder. This however imposes constraints on the type of interleaver that can be used.
The object of the present invention is to provide a receiver according to the preamble in which no constraints are imposed on the type of interleaver to be used.
To achieve said objective, the receiver according to the invention is characterized in that the receiver comprises a symbol error position selector for selecting from said possible symbol error positions a reduced number of possible symbol error positions and in that the symbol error position selector is arranged for providing the reduced number of symbol error positions to the decoder.
By reducing the number of possible error positions to be passed to the decoder, it is obtained that the error correcting capabilities of said decoder is not exceeded. This reducing is possible for each kind of interleaver to be used, resulting in that no constraints have to be imposed on the type of interleaver to be used.
An embodiment of the invention is characterized in that the symbol error position selector is arranged for selecting said reduced number of possible symbol error positions in dependence on a probability measure representing the probability that at a predicted error position the received symbol is actually wrong By passing only the symbol positions which are most likely to be wrong to the decoder, it is obtained that the most relevant positions are passed to the decoder and the less relevant are not. This results in an optimum decoding quality, given the numbers of predicted error positions which can be accepted by the decoder.
A further embodiment of the invention is characterized in that the burst error predictor is arranged for deriving the probability measure from errors present in previous words.
By deriving the probability measure from errors present in previous words, it is obtained that the probability measure is derived from actual statistical properties of the errors. This leads to a more accurate estimation of the probability measure, in particular when the statistical properties of the error measure change with time.